Recently I've noticed some smilie usage at the various forums that, while not being entirely incorrect, could be improved. Keep in mind that, as pictures are worth a thousand words, smilies can do wonders to convey emotion - but not facts. So, here are some tips:
1) Don't overuse smilies. There's nothing nearly so pointless and distracting as an excessive amount of smilies. Having too many reduces the value of each individual smilie. Take a look at the latter half of this post to see what I mean...
2) Use smilies to help convey emotion - I touched on this above. For example, if you're making a joke or being sarcastic, you might want to put in a smilie at the end to remind readers of that fact. I don't need to tell you how often miscommunication (that could have been avoided through the correct use of smilies) has resulted in flame wars and arguments on online forums.
3) Smilies should supplement other ideas. I usually overlook smilies if they appear in the middle of sentences and the like, which is where I often put them too - I've have to work on my placement. Use smilies at the end of paragraphs or on a new line, in order for readers to actually process the information.
4) Here is a brief summary of the various smilies. Sure, you say, some are obvious - for example, the normal smilie must mean "happy," right? Not really. In the following examples, spaces are used to separate characters so that you can see the actual characters without them being interpreted as smilies. Also note that the characters used for certain smilies and the images themselves vary from forum to forum.
: ) sympathetic - Use it when you want to express sincerity. This smilie, while not always applicable, can make someone feel very appreciated if it is used appropriately.
Example: Vincent, I hope your leg gets better soon.

: ( sad OR angry (it depends on the forum) - The sad smilie doesn't have much use because it rarely helps convey emotion.
Never use the angry smilie - all it does is generate controversy due to its negative implication, especially if directed towards another member.
Example: My Orca broke today.

(Already, a sad smilie doesn't add much to my message.)
Example: My Orca broke today.

(See how much ruder this sentence is than the previous one?)
: D huge smile (?) smilie - When used, this smilie conveys happiness, mild laughter, or perhaps acknowledgment of a joke. Be careful not to annoy anybody by using the smilie to laugh at something silly that he or she wrote.
Example: He tripped down the stairs!

(Funny, but with a touch of insensitivity.)
: P laugh smilie - This smilie is always used to laugh about something. It's similar to the smilie above, except this one is not so "smiley" and thus is a little easier to be interpreted as derogatory. Use it to laugh at your own mistake(s), not somebody else's. It can be enjoyable to see somebody think lightly of their own posts.
Example: He tripped down the stairs!

(Similar effect as with the previous smilie, but it's not even trying to be humorous.)
: rolleyes : rolling eyes smilie - This smilie takes the funny/laughing theme a final step further, removing most semblance of friendly laughter. Obviously, somebody giving the impression of rolling their eyes at somebody is extremely disrespectful. You may once again use it to laugh at yourself. Still, if in doubt, don't use this smilie.
Example: He tripped down the stairs!

(Downright patronizing to whoever was clumsy enough to trip.)
Example: I tripped down the stairs!

(I'm making fun of myself, which is okay. It's okay to laugh with somebody, but not at somebody.)
; ) wink smilie - Wink smilies are generally used if you give a tip to somebody. However, they can give an impression of inferiority, like with the rolling eyes smilie. And just for the record, IPB wink smilie is terrible - it doesn't really give an impression of winking, at least not at first glance. More like "hmm..." Just for reference, here's SSCentral vB wink smilie:

Example: Use a metal pipe nipple instead of an endcap for a homemade water gun nozzle if you want more range.

(Appropriate usage.)
Example: Dude, why didn't you use a metal pipe nipple for your nozzle?

(Part of the problem here is the wording, and not the smilie; but the smilie actually makes the negative emotion
worse.)
Example: Dude, why didn't you use a metal pipe nipple for your nozzle? (See, it's not as bad...)
B ) for IPB, : cool : elsewhere - cool smilie - I've preferred using this smilie of late. It has the same message as the wink smilie - suggesting a tip - but it seems to do so in a much friendlier way.
Example: Posting in dead topics is usually pointless because the people in question might no longer be active.

Example: Posting in dead topics is usually pointless because the people in question might no longer be active.

(See what I mean?)
I think that just about covers the more common smilies. I'll probably come back and make the layout of this post a bit easier to read.
I know people mean well, but using the wrong smilies or not using them at all can suggest insincerity or disrespect. This post also isn't directed at anybody; it just addressed an issue I've seen occasionally. Fortunately we've had no major issues at WWc, but things can always happen.